Not all wavelength regions of interest are directly accessible with lasers. Therefore, one or more laser beams of known wavelengths can be converted by the use of optical nonlinearities in one or more media to generate light with other wavelengths. For example, optical parametric oscillation can perform nonlinear conversion on commonly available laser sources to achieve optical power in the infrared. An optical parametric oscillator (OPO) is a coherent optical light source that operates based on nonlinear optical gain resulting from parametric amplification. This is in contrast to a laser that operates by stimulated emission in which incoming radiation can stimulate an excited laser ion to emit a photon into its own mode. An OPO device can consist of one or more nonlinear gain media contained within a resonant cavity that includes a partially reflecting output mirror to out-couple a portion of the newly generated light, while providing enough feedback to produce oscillation.
Various types of transparent crystalline materials can exhibit different kinds of optical nonlinearities associated with higher order complex nonlinear polarization components. Frequency conversion is one consequence of higher order polarization components. Difference frequency generation (DFG), a second order effect, is the fundamental process exploited in an OPO. Within the non-linear medium the parent laser beam induces a driving polarization wave, which generates two new beams called ‘signal’ and ‘idler’. The resonant signal interacts with the driving polarization to set up an idler polarization at the difference frequency. The difference in phase velocity between the uncoupled freely propagating idler field and the driving polarization due to material dispersion causes a relative phase slip along the propagation direction. Because of this relative phase slip, the direction of energy flow between the idler and driving polarization oscillates as they propagate through the material. Hence, the generated fields can not grow continuously. The physical distance over which power flows positively from the driving wave to the signal and idler is called a coherence length Lc. Phase matching techniques can be used to compensate for the phase slip and increase the effective coherence length to encourage positive energy flow from the pump beam to the signal and idler. Increasing the coherence length allows the oscillating signal field to grow as it constructively interacts with the pump's driving polarization over longer distances.
As one example, birefringent crystals can be used as nonlinear gain media, satisfying the phase matching condition to convert in the infrared through the process of birefringent phase matching (BPM). The two main categories of operation for BPM devices are Type I and Type II. Type I phase matching generates two parallel polarized beams called ‘signal’ and ‘idler’, which are orthogonal to the parent beam polarization. Type II phase matching generates one beam with polarization parallel to the source beam, and another beam that is orthogonal to the source beam.
Quasi-phase matching (QPM) is a technique of using spatially modulated nonlinear properties of a gain medium. By periodically rotating the crystal orientation about the appropriate propagation axis by π every coherence length, QPM also reverses the non-linearity and compensates for the phase slip. The waves are then phase matched for efficient conversion. A QPM crystal can be engineered to exploit its highest nonlinearity, while avoiding walk off due to non-critical phase matching which occurs in critically phase matched BPM materials. The operating wavelengths of QPM materials can also be engineered and tuned over a wide range of wavelengths.
Many applications require narrow linewidths that are technically not commercially feasible with many OPO or laser devices, including those implementing Type I BPM or QPM. For example, narrow linewidths are often required for pump sources for nonlinear processes, for light sources for various kinds of fiber optic sensors, for spectroscopy, in coherent optical fiber communications, and for test and measurement.